Passive, two-phase (liquid/vapor) heat transfer devices, including several types of heat pipes and thermosyphons, are generally constant temperature heat transfer devices. A schematic of these devices in accordance with prior art is presented in FIG. 1. Heat released from a hot air stream 105 is transferred to the cool airstream 107 by the heat pipe or thermosyphon. Vapor 110 flows in the center of the heat pipe, and liquid 111 flows on the inside perimeter, and is driven by gravity (for thermosyphons) or capillary action (for heat pipes). Heat pipes and thermosyphons, in accordance with prior art, are designed to operate as constant temperature devices. This can introduce design problems, as coolants, like air, which remove heat from heat pipes and thermosyphons, use sensible energy, thus requiring a change of temperature to absorb and release heat.
In order to get a counter-flow heat exchanger effect between hot 105 and cold 107 fluids, several heat pipes are necessary within the same heat exchanger. The need for several individual heat pipes can have the effect of increasing costs and the complexity of the system integration. The complexity can increase significantly, as the distance between the hot 105 and cold 107 fluids streams increases. Additionally, if there is a desire to control the heat flow rate by valves, the number of valves necessary, scales with the number of heat pipes. These attributes can severely limit conventional heat pipes and thermosyphons from many practical applications.